The Facts on EHV-1

Recognize the signs of equine herpesvirus-1 and learn how to protect your horse from the risks of the virus.

Recognize the signs of equine herpesvirus-1 and learn how to protect your horse from the risks of the virus.

As many as 80 percent of horses become latently infected with EHV-1 (the virus becomes inactive in the lymph nodes) and serve as reservoirs for the disease. (Journal photo).

By Dr. Thomas R. Lenz in The American Quarter Horse Journal

Although there are a number of herpesviruses that can affect horses, mules and donkeys, equine herpesvirus types 1(EHV-1) and 4 (EHV-4) are the two we’re most concerned with. EHV-4 produces the majority of upper respiratory infections we see in weanlings and yearlings and very rarely causes abortion in mares or neurological disease. EHV-1, on the other hand, causes respiratory disease, abortion in mares, neonatal foal death and/or neurologic disease.

The neurologic form of EHV-1 is equine herpesvirus myeloencephalopathy and is the form of the disease in the May outbreak that was seen in the western part of the country. According to the United States Department of Agriculture in their final EHV-1 Situation Report from June 22, there were 90 confirmed cases of EHV/EHM in 10 states (Arizona, California, Colorado, Idaho, New Mexico, Nevada, Oklahoma, Oregon, Utah and Washington).

Of the 90 confirmed EHV/EHM cases, 54 were cases that were at the Ogden, Utah, event April 29 to May 8. Thirteen horses associated with the outbreak died or were euthanized. Nine of those horses attended the show in Utah. The fast action by state veterinarians and show officials to limit the travel of horses, kept the development of few new cases quite low.

EHV is a tricky virus. When young horses first contract it, they experience fever, lethargy, clear nasal discharge, watery eyes and occasionally a cough. Because the virus does not stimulate a strong immune response, they might come down with EHV-1 many times during their life, but each time they do, they experience milder symptoms.

As many as 80 percent of horses become latently infected (the virus becomes inactive in the lymph nodes) and serve as reservoirs for the disease. When the latently infected horse is stressed by trailering, a horse show, intense training, etc., it might start shedding the virus without showing clinical signs, making containment of the virus very difficult.

Learn more about your horse’s health with AQHA’s Common Horse Health Issues report.

Most horses only experience the respiratory form of EHV-1, but in very rare cases the disease progresses into neurological disease. These horses are usually stressed, are very young or very old, and might have compromised immune systems.

The incubation period of EHV (time from exposure to becoming ill) is one to two days with clinical signs of fever, depression, nasal discharge and loss of appetite. If the neurological form of the disease occurs, it usually does so within eight to 12 days and the horse develops clinical signs of weakness, lethargy, urine dribbling, decreased tail tone and the inability to stand.

In addition to EHV, the most likely other upper respiratory infective agent is equine influenza virus, which will also cause an elevated temperature, depression and nasal discharge, but will not progress to EHM.

Unlike equine influenza virus that is spread by coughing, EHV-1 is spread through direct contact with infected horses as well as contaminated water buckets, feed tubs, tack, grooming equipment and even on the hands of people.

Biosecurity is key to disease prevention. Particular attention should be given to cleaning and disinfecting stalls and equipment periodically. Isolate sick horses or new arrivals on your premises for at least 21 days to ensure they are not infective.

When you arrive at a horse show or stable, you have no idea of what diseases the previous occupant might have been shedding, so it is a good idea to disinfect the stall before putting your horse in it. Manure and dirt can make bleach-based disinfectants (one part bleach to 10 parts water) ineffective, so it’s best to use Nolvasan or Roccal-D. Avoid nose-to-nose contact with other horses, and do not water your horses at communal water tanks. Discourage visitors from petting the noses of your horses or other horses, as they can spread the virus on their hands. It’s a good idea to provide waterless hand sanitizers. (See more tips on everyday biosecurity in the July 2011 issue of The American Quarter Horse Journal.)

Check your horse’s temperature each morning as most respiratory infections, especially EHM, are preceded by an elevated body temperature (102.5 degrees Fahrenheit or more).

If one of your horses becomes sick, isolate it immediately and contact your veterinarian, who will take nasal swabs and blood samples to help identify the cause of the infections and develop an action plan.

Keep up to date Coggin’s papers and health certificates.

AQHA’s Common Horse Health Issues report gets you steered in the right direction and arms you with the knowledge you need to be a responsible horse owner. Download your copy today!

Don’t share equipment, and limit your horse’s exposure to other horses and disease vectors like mosquitoes or flies. Horses that travel to events where horses will be concentrated should be vaccinated against EIV, EHV-1, and EHV-4 at least twice yearly to prevent respiratory infection.

There are no vaccines currently on the market that are labeled as an aid in preventing EHM. However, a number of studies have shown that high antigenic mass EHV-1 vaccines, like those used to prevent abortion in broodmares, decrease viral shedding and the number of virus particles in the bloodstream so it is a good idea to booster your horses with one of them and place the horses on an immunomodulator 10-14 days prior to leaving for the event.

As always, your local veterinarian is the best source of information in your area so ask him to help you develop a good vaccination and preventive medicine plan for your horses. To keep up with the status of the current EHV outbreak, go to

In the News

On September 15, a horse exhibiting clinical signs of EHV-1 was brought to the University of Tennessee Veterinary Medical Center. The horse was euthanized and test results confirmed the horse was positive for the neurological strain of EHV-1. For more information concerning the case, visit released another article on September 16, which detailed the case of nine California horses confirmed EHV-1 positive.

A 23-year-old Thoroughbred mare was euthanized on September 1 in Richland, Michigan, after she exhibited clinical signs of the neurological strain of EHV-1. Tests later confirmed that the mare was EHV-1 positive.

EHV-1 Tips

Check out these links for tips on how to detect EHV-1 and how to protect your horses against the virus:

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